-LRB- CNN -RRB- -- Authorities investigating the case of a boy who disappeared in Kansas almost a decade ago plan to search an undisclosed residence Wednesday , the Butler County sheriff said .

An age-progression photo shows what Adam Herrman would like today , as a 21-year-old man .

Sheriff Craig Murphy would not disclose details about the residence or why authorities want to search it .

He said his department will also search on an area of the Whitewater River , in southern Kansas , on Saturday near where Adam Herrman was last seen .

Adam was 11 when he went missing in 1999 . He was living in a mobile home park in Towanda , a town about 25 miles northeast of Wichita , with his adoptive parents , Doug and Valerie Herrman , authorities said .

Wichita attorney Warner Eisenbise , who is representing Adam 's adoptive parents , said the couple believed Adam had run away and did n't report him missing . They `` really rue the fact that they did n't '' report him missing , he said Monday .

A few weeks ago , an undisclosed person contacted the Wichita-Sedgwick County Exploited and Missing Child Unit , expressing concern about Adam , the sheriff said .

The Herrmans told Eisenbise that Adam ran away frequently , the attorney said , and they believed he was either with his biological parents or homeless . Although the Herrmans did not report him missing , `` they were very worried about him , '' Eisenbise said .

In an interview published Tuesday in The Wichita Eagle , Valerie Herrman said Adam ran away in May 1999 after she spanked him with a belt . She said she was upset but does n't remember why , The Eagle reported .

The couple never reported Adam missing , Valerie Herrman told the paper , because they feared authorities would take Adam and his siblings away because of the spanking .

The couple adopted his two younger siblings as well , according to The Eagle .

`` We love him , and we made a terrible mistake '' by not reporting him missing , Doug Herrman told The Eagle . The couple said they searched the mobile home park and other areas for two days after Adam left .

`` Then we came to the conclusion that the police probably have him , and they 're coming to us , probably to get us in trouble , '' Doug Herrman told the newspaper , but the `` police never came . ''

Authorities have searched an empty lot in the Pine Ridge Mobile Home Park where the family lived . There , police found an `` answer '' to one of their questions , Murphy said Monday without elaborating .

Eisenbise said that on December 15 , authorities also searched the Herrmans ' homes in Derby , outside of Wichita , and took the couple 's computer , he said .

The National Center for Missing and Exploited Children has released an age-progression picture that depicts Adam as he might appear now : a young man with blue eyes and light-colored hair .

Adam had been placed in the Herrmans ' care when he was about 2 , Murphy said Monday .

He had been named Irvin Groeninger III when he was born June 8 , 1987 , Murphy said , and it was not clear when his name was changed . His biological parents relinquished their rights as parents about two decades ago , and Adam and his siblings were put in foster homes , CNN affiliate KWCH reported .

`` I thought what I was doing for them was in the best interest of the children , and evidently it was n't , '' Irvin Groeninger , Adam 's biological father , told KWCH . `` If he was still in my custody , this would have never happened . ''

Adam 's sister , Tiffany Broadfoot , 22 , said she had last seen her brother about 14 years ago at a birthday party .

`` He had the cutest little round face , little-bitty freckles right up here on the tip of his cheek , '' she told the station .

CNN 's Taylor Gandossy contributed to this report .

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Sheriff says his officers will search area near Whitewater River on Saturday

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Attorney says parents `` rue the fact '' they did n't report him missing

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Adam Herrman reportedly ran away when he was 11 , after being spanked

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Parents tell paper they thought spanking would lead police to take other kids away